Observation of the type of hydrogen bonds in coal by FTIR

It is hard to distinguish the type of hydrogen bonds formed by hydroxyl groups by normal FTIR methods because of the interference of water in samples. In this work, a method is proposed for the clear observation of the types of hydrogen bonds by FTIR. The method involves the evaporation of water in a vacuum chamber with CaF2 windows (vacuum level, 4 x 10(-3) kPa) followed by in situ scanning of the IR spectrum. Five types of hydrogen bonds were found in coal and its extracts. The hydroxyl groups in coal mainly exist as self-associated OH and OH-N hydrogen-bonded pairs. The thermal stability of these hydrogen bonds was investigated by means of in-situ pyrolysis FTIR. It follows the order of OH-ether O > self-associated OH x cyclic OH > OH-N > OH-pi.